In recent years, electroluminescent devices have been attracting attention as, for example, large-area solid state light sources to replace incandescent lamps and gas-filled lamps. They have also been attracting attention as self-luminous displays, and are the most promising alternative to liquid crystal displays in the flat panel display (FPD) field. In particular, an organic electroluminescent (EL) device, in which the device material is formed from an organic material, is being commercialized as a low power consumption full-color FPD.
Conventionally, organic EL devices generally produce phosphorescence when an excited singlet relaxes to the ground state. However, the proportion of excitons generated by injection of charge into an organic film is said to be statistically singlet : triplet=1:3, and the theoretical limit value of the internal quantum efficiency of an organic EL device is said to be 25%. This is one of the hurdles to be overcome when lowering the power consumption of an organic EL device.
As one means for solving this problem, a device utilizing phosphorescence from an excited triplet has been investigated. If phosphorescence from an excited triplet can be utilized, it can be expected that in principle the luminescence quantum yield would be at least three times that obtained when fluorescence from an excited singlet is utilized. Furthermore, while taking into consideration utilization of an exciton resulting from intersystem crossing from the singlet, which has high energy, to the triplet, it can be expected that in principle the luminescence quantum yield would be four times greater, that is, it would be 100%.
Examples of research that has been carried out so far include M. A. Baldo et al., Appl. Phys. Lett. 1999, 75, 4. In this publication, the materials below are used. The materials are abbreviated as follows.    Alq3: an aluminum-quinolinol complex (tris(8-quinolinolato)aluminum)    α-NPD: N,N′-Di-naphthalen-1-yl-N,N′-diphenyl-biphenyl-4,4′-diamine    CBP: 4,4′-N,N′-dicarbazole-biphenyl    BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline    Ir(ppy)3: iridium-phenylpyridine complex (tris(2-phenylpyridine)iridium)
Examples in which luminescence from a triplet is utilized include Japanese Patent Application Laid-open Nos. 11-329739, 11-256148, and 8-319482.